1. Field of the Disclosure
A method for twisting hollow bars into a desired longitudinal twisted spiral form is disclosed. The hollow bars may form part or all of a grille, railing, metalwork and like architectural element where the twisted spiral form may serve as decorative or ornamental purposes. A number of components or parts for use in respect of twisting the hollow bars are also disclosed.
2. Discussion of the Background Art
In addition to providing barrier purposes, the row of bars forming a grille, grating, divider, railing, and other metalwork pieces, openwork cover or barrier to doors and windows, louvre, lattice forms and the like architectural or furniture elements may be treated to improve upon its otherwise spartan barricading outlook. In particular, each of the bars may be purposely deformed, or so formed, into certain twisted pattern along its longitudinal axis resulting in a twisted or helical spiral for aesthetics or ornamentation.
An example of such twisting imparted to a bar or strip by an apparatus is disclosed in U.S. Pat. No. 5,107,694 (Kemp). As the strip of material or thin bar is solid, this prior art apparatus may use roll-milling to form the helical twists on the bar. Often it is not necessary for the bar to be solid in order to provide the necessary tensile strength such as that demonstrated by I-beam (or H-beam) cross-section. It is also known that while the flanges (horizontal elements) of the I-beam is very efficient in carrying both bending and shear in the plane of the web (vertical element), it is inefficient in carrying torsion for which hollow structural sections are preferred.
Methods for twisting hollow or tubular elements known in the art includes that disclosed in U.S. Pat. No. 5,410,808 (Geppelt et al.) for twisting a hollow double wall tube (i.e. bar having cylindrical cross-section). Prior art methods for twisting hollow rods having a rectangular cross-section, include U.S. Pat. No. 5,771,726 (Bibby et al.) which discloses a method and apparatus for twisting a hollow rod having open seam rectangular profile for use as curtain railing. Apart from the telescopic feature allowing the curtain rod to be lengthened with the same twisting aesthetics at the ends, this Bibby reference also mentions the problem of open-seam bars as having greater resilence to twisting due to the tendency of such bars to spring back partially or wholly to its untwisted state upon release from the twisting apparatus, or when the moment of force is removed, compared to close-seam or seam-less or tubular bars.
For tubular bars, the twisting is often irreversible and once twisted the material becomes permanently deformed once its elasticity point is breached. Any subsequent working on the bar would thus lead to material fatigue or material failure. It is thus desirable to maintain certain resilience in fabricating twisted bars so that the twisting may be reversed to a certain extent and re-twisted to adjust for fitting length, asthetics as well as without compromising tensile or structural strength and so that the bar being worked on need not be discarded if a slight workmanship error occurs or a small specification readjustment is required. Of course, it is well-known that material resilience and recovery from deformity depends on the material employed, such as whether the hollow bar is made of metal, alloy or polymers (including plastics) and its material properties such as its elasticity, torsional, shearing and tensile strengths, etc.